This invention relates to vapor generators, and more particularly, to vapor generators of the natural circulation type.
In a natural circulation vapor generator a vaporizable fluid, such as water, is passed through a group of evaporator tubes, or riser tubes which are disposed within, or are arranged to form, the furnace walls of the vapor generator. A fuel is burned within the furnace, thereby giving off heat which is absorbed by the vaporizable fluid passing through the riser tubes. As the fluid is heated, a vapor-liquid mixture rises through the riser tubes and passes into a separating drum which communicates with the outlet ends of the riser tubes. Saturated vapor is separated from the liquid within the drum, and passed to a superheater inlet or a point of use. Saturated liquid, the liquid phase of the vaporizable fluid passed to the drum, is returned through unheated downcomers to the inlets of the riser tubes. The pumping head in this type of vapor generator is provided by the density difference between the saturated liquid in the unheated downcomers and the vapor-liquid mixture in the heated risers.
One of the many factors to be considered in designing a vapor generator is the type of fuel to be burned. If a fuel different from the one considered during design is used, then the vaporizing circuits of the vapor generator may not function as intended due to a heat absorption pattern different from the pattern for which the unit was designed. In view of present shortages of natural gas and oil, it has become necessary to convert existing vapor generators to allow for burning coal. It is therefore possible that after conversion unfavorable heat patterns may be exhibited.
Change of fuel is only one reason why an unfavorable heat pattern may be experienced, however. Improper design, as well as a change in the mode of operation, going from base load to cycling load, for example, may also contribute to unfavorable heat patterns.
The instant invention is intended to improve the circulation for less than favorable heat absorption patterns of a natural circulation fossil fuel fired vapor generator by providing a water jet pump to enhance the circulation in the vaporizer circuits.